Liquid crystal material is a material in a liquid crystalline state which is between the solid state and the liquid state. The phase states of a liquid crystal material can be roughly classified into nematic phase, smectic phase and cholesteric phase. Liquid crystal materials can be classified into dynamic scattering mode (DS), guest-host mode (GH), twisted nematic mode (TN), super twisted nematic mode (STN), vertical alignment mode (VA) and the like. Liquid crystal elements formed by sandwiching the liquid crystal composition between two electrodes can be widely used in display devices such as electronic calculators, various test instruments, car dashboards, computers, laptops, mobile phones, televisions and the like, and dimming devices such as 3D glasses, motor vehiclrearview mirrors, light valves for welding, camera viewfinders and the like, due to the specific electro-optics properties thereof.
Linear polarizers are required for most liquid crystal elements for obtaining polarized light, resulting in utilization of light inefficient. Due to the dichroism (also known as absorption anisotropy, i.e. different absorbances are shown to polarized light perpendicular to or parallel to the absorption axis of the dichromatic dye) derived from the dichromatic dye, the liquid crystal elements comprising the dichromatic dye contained in a liquid crystal composition (i.e. a guest-host liquid crystal composition) can realize monochrome or multi-color display with fewer or without any polarizer, has higher luminance and display contrast and wide viewing angle, and can eliminate chromatic aberration. In particular, light emitting display can also be realized by the liquid crystal elements in reflection mode without backlight, and the utilization of light is efficient.
Most dichromatic dyes are anthraquinone dyes or azo dyes. A single dichromatic dye mainly absorbs the light with particular wavelength and shows the complimentary color of the transmit light, since different substituent groups in the dye molecules show different absorption characteristics to visible light. For example, dyes mainly absorbing red, yellow, green and blue colored light respectively will show the colors of blue, purple, red and yellow, respectively. Therefore, it is difficult for a single dichromatic dye to show a color of black. Therefore, mixtures of multiple dyes are required to obtain an absorbance to the visible light that is a constant; meanwhile, according to the sensitivity of human eyes to light, only a uniform absorption to the light with wavelength ranged from 420 nm to 680 nm can create a color called black.
If the absorption to light occurs when the direction of vibration of the polarized light is consistent with the direction of long axis of the dye molecule, the dye can be called a positive dye (P-type dye). If the absorption to light does not occur when the direction of vibration of the polarized light is consistent with the direction of long axis of the dye molecule, the dye can be called a negative dye (N-type dye). Since the species of dichroism N-type dyes only are half of that of the dichroism P-type dyes, generally, most are P-type dyes.
As for the guest-host liquid crystal composition comprising dichromatic dye, the more uniformly it absorbs the light with wavelength ranged from 420 nm to 680 nm, the more uniformly the transmittance curve distributes, and thus the better effect of displaying the color of black the display device gets. Therefore, a dichromatic dye is required to exhibit good and uniformly absorption property to the light with wavelength ranged from 420 nm to 680 nm and the properties such as high solubility and the like. Relative to the properties such as contrast, color reproduction and the like of the guest-host liquid crystal composition, transmittance and solubility of the dichromatic dye in the liquid crystal have a great effect on the display quality of guest-host display device. The contrast can be improved by increasing the concentration of the dichromatic dye at the cost of light emission display, a characteristic of guest-host display device. Therefore, the above two characteristics, i.e. low transmittance and high solubility, are required to be maintained in both dichromatic dye and host liquid crystal. However, transmittance of the dichromatic-dye-contained liquid crystals currently commercially available is higher, and distribution of the transmittance curve thereof is nonuniform, and thus it is difficult to arrive at a good effect of displaying the color of black.
Therefore, a dichromatic dye composition with low transmittance and good miscibility is in particular need in liquid crystal material art. In particular, in order to arrive at the effect of displaying the color of black, a dichromatic dye composition having good absorption to lights with wavelength ranged from 420 to 680 nm is needed.